Why do ants surround sperm?

I assume by sperm you mean the ejaculate of a mammal. If so, suppose some of the fluid fell where any scavenging species of ant, or many species of meat-eating ant, should happen to find it. Then, especially if it still is moist, and the place is dry, the ants would simply regard it as a protein-rich, nutritious fluid. They would behave similarly if they found a drop of milk or blood or something similar. To them it would be about as valuable as finding a drop of honey, or even more valuable than honey if they were short of protein rather than sugar. The ant that discovered it would fill its crop and carry it back to the colony, where it either would store the food or share it with its fellow-workers, or feed it to the larvae. Passing on food in such ways sometimes is called trophallaxis. It then would return to get more, and if it happened to be a species of ant that recruits fellow-workers when it finds food, it would use some form of signal that would cause them to follow her to the food. Pretty soon the droplet would be surrounded by ants, and they would keep busy till they finished it or they had had enough.